A surface-emitting laser, such as a vertical cavity surface emitting laser (VCSEL) is a semiconductor laser that emits light in a direction perpendicular to a substrate. Compared to an edge-emitting semiconductor laser, the surface emitting laser is low-cost and high-performance, and can be easily implemented in an array. Thus, there is a growing expectation that the surface emitting laser will provide light sources for optical communications technologies, such as for optical interconnections, optical pick-ups, and image forming apparatus such as laser printers. Vigorous research and development of the surface emitting laser technology is underway, and some of the technology has already been put to practical use.
In recent years, a surface-emitting laser element has been proposed in which an optically transparent dielectric film is formed on an emitting surface, and a high-order transverse mode is controlled by providing a difference in reflectivity between a central portion and a peripheral portion of the emitting surface. Another surface-emitting laser element has also been proposed in which the polarization direction is further stabilized by providing shape anisotropy in such a dielectric film (see Patent Documents 1 through 4).
Generally, when a laser element that emits laser light is used with an optical system, light reflected by a lens or glass surface in the optical system may become incident on the laser element as feedback light, thereby varying the amount of emitted laser light. Thus, laser elements resistant to such feedback light have been proposed. For example, Patent Document 5 discusses a surface-emitting laser element in which a resonator is formed by a lower multilayer mirror and an upper multilayer mirror, where the relaxation oscillation frequency at a bias point in the resonator is set above an optical communication frequency for modulating the laser light emitted by the surface-emitting laser element.
Patent Document 6 discusses a surface-emitting semiconductor laser comprising a semiconductor substrate. An active layer is disposed on top of the semiconductor substrate, and an emitting plane disposed on top of the active layer is configured to emit the laser light produced in the active layer in a vertical direction with respect to the semiconductor substrate. An absorption layer is disposed on top of the emitting plane and configured to absorb some of the laser light.
It has also been proposed to control the feedback light itself. For example, Patent Document 7 discusses a surface-emitting laser module for optical transmission comprising a TO header on which at least a surface-emitting laser chip and a monitoring photodetector are mounted. The module includes a cap with a window coated with a film having a transmittance of 40% or less.
However, in the surface-emitting laser element according to Patent Document 5, the structure of a drive circuit as well as the element itself is complex, resulting in increased cost. The surface-emitting semiconductor laser according to Patent Document 6 also has a complex element structure and cannot provide sufficient effects. When the surface-emitting laser module according to Patent Document 7 is applied to a surface-emitting laser array, the angle of inclination of the window glass needs to be increased, making it difficult to process the cap and also resulting in an increase in size.    Patent Document 1: JP 2001-156395A    Patent Document 2: JP Patent No. 3955925    Patent Document 3: JP 2007-201398A    Patent Document 4: JP 2004-289033A    Patent Document 5: JP 2005-252032A    Patent Document 6: JP 2005-086027A    Patent Document 7: JP 2007-103576A